1. Field of Invention
The present invention relates to an electron beam generator and more particularly to such a generator for employment in the fabrication of semiconductor devices.
2. Description of Prior Art
Integrated circuits are created of different active elements fabricated within or on the surface of a crystalline substrate. This fabrication is accomplished by a series of processes for forming areas of different conductivities, in or on the surface of the substrate by the diffusion of impurity atoms into those areas so as to form the active elements. In addition to diffusion, ion implant and epitaxial growth are the standard methods of creating such active elements.
The formation of such an integrated circuit chip and a number of such chips in a crystalline wafer require a number of process steps such as masking the crystalline surface and exposing only those areas in which impurity atoms are to be diffused or implanted or epitaxial areas are to be grown, covering such areas with an insulating or oxide material and so on. In each of these steps, the pattern to be formed is created by covering the surface of the crystalline wafer with a photo resist material and optically exposing that surface through a mask containing the pattern to be created. Such masks are in turn created by drawing the pattern on an enlarged scale and photographically reducing it to the size of the elements which are to be formed in the integrated circuit chip.
The active elements which can be formed in an integrated circuit chip by the above described processes generally are not any smaller than 2 microns in dimension and it is believed that the resolution that can be obtained by such optical lithography processes has reached its limit. Electron beam lithography methods have been employed to achieve higher resolution but have not been greatly successful because of the time involved to generate the patterns due to the low intensity of the electron beam which requires long exposure times for the electron beam resist material employed.
An additional disadvantage of prior art electron beam generators is that the focusing lens was placed relatively close to the target which allowed for faster scanning of the target but only over a relatively small area. Thus, the scan cycle had to be repeated many times in order to cover the entire target area.
With a sufficiently improved electron beam column having a high beam current density and a large image focal length, the entire target chip area can be scanned and the pattern, to be imposed thereon, generated much faster than with prior art electron beam generators. Furthermore, not only can such an electron beam generator be employed to form high resolution masks for use in integrated circuit fabrication but the respective steps of masking can be eliminated and the pattern can be generated directly on the surface of the integrated circuit wafer.
It is, then, an object of the present invention to provide an improved electron beam generator.
It is still another object of the present invention to provide an electron beam generator for employment in semiconductor manufacture which generator produces beams of high current densities.
It is still another object of the present invention to provide an electron beam generator having a focusing mechanism so arranged as to provide a large image focal distance to accomodate the scanning of larger target areas.